Thermal ionization utilizes resistive heating of a filament to desorb and spontaneously ionize elemental species from a solid sample located in contact with the filament. In an analytical protocol, the desorbed ions are collected via acceleration and focusing to form an ion beam that is directed to a mass spectrometer. Thermal ionization mass spectrometry (TIMS) is the benchmark technique for determination of isotope ratios of elements in geochronology and tracer studies. For example, TIMS is commonly utilized in analysis of radiometric systems including U→Th→Pb, Rb→Sr, Sm→Nd, Lu→Hf, and the uranium series disequilibrium. TIMS is also useful in analysis of non-terrestrial systems in determining the decay of short-lived radionuclides as found in meteorites such as Fe→Ni, Mn→Cr, Al→Mg, etc. Non-radiogenic, stable isotope ratios for various elements such as Li, B, Mg, Ca, and Fe are also often characterized by use of TIMS in order to, e.g., characterize exchange processes, track reservoir interaction and evaluate kinetic processes.
While TIMS offers many benefits to analytics including very precise measurements, consistent mass fractionation, highly automated operation, and near 100% transmission of ions from the source to the collector, disadvantages exist. One particular issue that continues to plague TIMS is low ionization efficiency at the ion source. For many elements (e.g. uranium and plutonium) only a very small fraction of the analytical sample, on the order of 0.1% to 0.2%, is ionized and subsequently detected. Other disadvantages of TIMS are directly or indirectly tied to the low ionization efficiency of the systems such as limits on sample materials, with a difficulty in using the technique with elements that exhibit large first ionization potentials; and a requirement for extensive sample preparation in order to obtain pure enough samples such that ionization is not affected by contaminate elements or isobars.
What are needed in the art are systems and methods that can increase the ionization efficiency during thermal ionization of a sample. In particular, systems and methods that incorporate thermal ionization filaments formed of materials and/or having a geometry that can increase efficiency of the system would be of great benefit. This would be particularly beneficial for expanding the capabilities of TIMS.